Such a therapy system is known from the paper ‘Local hyperthermia with MR-guided focused ultrasound: spiral trajectory for the focal point optimized for temperature uniformity in the target region’ by R. Salomir et al. in J. Magn. Res. Im. 12(2000)571-583. This paper discloses a therapy system which is an MR-guided focused ultrasound system. The therapy module is formed by a spherical ultrasound transducer of which the focus is moved along a double inside-out spiral trajectory covering the target region. The intensity of the acoustic field of the spherical ultrasound transducer has approximately a Gaussian distribution around the focal point. A magnetic resonance imaging system provides both anatomical information for target definition and temperature maps. The cited paper mentions to employ fast MR-thermometry and on-line feedback to the ultrasound apparatus. A feedback algorithm is based on real-time evaluation of temperature gradients around the focal point. In particular, the focal spot is moved over the spiral trajectory under continuous and maximum focused ultrasound power. Differences between the real and a target temperature during the first spiral are corrected during the second spectral trajectory.